The cell is the basic building block of living organisms. Bacteria and the
parasite that causes malaria consist of single cells, while plants and animals
are made up of trillions of cells. Most cells are spherical or cube shaped but
some are a range of different shapes (see diagram 3.1).
Most cells are so small that a microscope is needed to see them, although a
few cells, eg the ostrich�s egg, are so large that they could make a meal for
several people.
A normal cell is about 0.02 of a millimetre (0.02mm) in diameter. (Small
distances like this are normally expressed in micrometres or microns (�m). Note
there are 1000 �ms in every mm).
Diagram 3.1. A variety of animal cells
When you look at a typical animal cell with a light microscope it seems quite
simple with only a few structures visible (see diagram 3.2).
Diagram 3.2 An animal cell
Three main parts can be seen:
-
- an outer cell wall or plasma membrane,
- an inner region called the cytoplasm and
- the nucleus
However, when you use an electron microscope to increase the magnification
many thousands of times you see that these seemingly simple structures are
incredibly complex, each with its own specialized function. For example the
plasma membrane is seen to be a double layer and the cytoplasm contains many
special structures called organelles (meaning little organs) which are
described below. A drawing of the cell as seen with an electron microscope is
shown in diagram 3.3.
Diagram 3.3 An animal cell as seen with an electron microscope
The Plasma Membrane
The thin plasma membrane surrounds the cell, separating its contents from the
surroundings and controlling what enters and leaves the cell. The plasma
membrane is composed of two main molecules, fats (in fact phospholipids) and
proteins. The fats are arranged in a double layer with the large protein
molecules dotted about in the membrane (see diagram 3.4). Some of the protein
molecules form tiny channels in the membrane while others help transport
substances from one side of the membrane to the other.
Diagram 3.4 The structure of the plasma membrane
How substances move across the Plasma Membrane
Substances need to pass through the membrane to enter or leave the cell and
they do so in a number of ways. Some of these processes require no energy i.e.
they are passive, while others require energy - they are active, while others
require energy i.e. they are active.
Passive processes include: a) diffusion and b) osmosis, while active
processes include: c) active transport, d) phagocytosis, e) pinocytosis and f)
exocytosis. These will be described below.
a) Diffusion
Although you may not know it, you are already familiar with the process of
diffusion. It is diffusion that causes a smell (expensive perfume or smelly
socks) in one part of the room to gradually move through the room so it can be
smelt on the other side. Diffusion occurs in the air and in liquids.
Diagram 3.5 shows what happens when a few crystals of a dark purple dye
called potassium permanganate are dropped into a beaker of water. The dye
molecules diffuse into the water moving from high to low concentrations so they
become evenly distributed throughout the beaker.
Diagram 3.5 - Diffusion in a liquid
In the body, diffusion causes molecules that are in a high concentration on
one side of the cell membrane to move across the membrane until they are present
in equal concentrations on both sides. It takes place because all molecules have
an in-built vibration that causes them to move and collide until they are evenly
distributed. It is an absolutely natural process that requires no added energy.
Small molecules like oxygen, carbon dioxide, water and ammonia as well as
fats, diffuse directly through the double fat layer of the membrane. The small
molecules named above as well as a variety of charged particles (ions) also
diffuse through the protein-lined channels. Larger molecules like glucose attach
to a carrier molecule that aids their diffusion through the membrane. This is
called facilitated diffusion.
In the animal�s body diffusion is important for moving oxygen and carbon
dioxide between the lungs and the blood, for moving digested food molecules from
the gut into the blood and for the removal of waste products from the cell.
b) Osmosis
Although the word may be unfamiliar, you are almost certainly acquainted with
the effects of osmosis. It is osmosis that plumps out dried fruit when you soak
it before making a fruit cake or makes that wizened old carrot look almost like
new when you soak it in water. Osmosis is in fact the diffusion of water across
a membrane that allows water across but not larger molecules. This kind of
membrane is called a semi-permeable membrane.
Take a look at side A of diagram 3.6. It shows a container divided
into two parts by an artificial semi-permeable membrane. Water is poured into
one part while a solution containing salt is poured into the other part. Water
can cross the membrane but the salt cannot. The water crosses the semi-permeable
membrane by diffusion until there is an equal amount of water on both sides of
the membrane. The effect of this would be to make the salt solution more diluted
and cause the level of the liquid in the right-hand side of the container to
rise so it looked like side B of diagram 3.6. This movement of water
across the semi-permeable membrane is called osmosis. It is a completely natural
process that requires no outside energy.
Diagram 3.6 - Osmosis
Although it would be difficult to do in practice, imagine that you could now
take a plunger and push down on the fluid in the right-hand side of container
B so that it flowed back across the semi-permeable membrane until the level
of fluid on both sides was equal again. If you could measure the pressure
required to do this, this would be equal to the osmotic pressure of the
salt solution. (This is a rather advanced concept at this stage but you will
meet this term again when you study fluid balance later in the course).
The plasma membrane of cells acts as a semi-permeable membrane. If red blood
cells, for example, are placed in water, the water crosses the membrane to make
the amount of water on both sides of it equal (see diagram 3.7). This means that
the water moves into the cell causing it to swell. This can occur to such an
extent that the cell actually bursts to release its contents. This bursting of
red blood cells is called haemolysis. In a situation such as this when
the solution on one side of a semi-permeable membrane has a lower concentration
than that on the other side, the first solution is said to be hypotonic
to the second.
Diagram 3.7 - Osmosis in red cells placed in a hypotonic solution
Now think what would happen if red blood cells were placed in a salt solution
that has a higher salt concentration than the solution within the cells (see
diagram 3.8). Such a bathing solution is called a hypertonic solution. In
this situation the �concentration� of water within the cells would be higher
than that outside the cells. Osmosis (diffusion of water) would then occur from
the inside of the cells to the outside solution, causing the cells to shrink.
Diagram 3.8 - Osmosis in red cells placed in a hypertonic solution
A solution that contains 0.9% salt has the same concentration as body fluids
and the solution within red cells. Cells placed in such a solution would neither
swell nor shrink (see diagram 3.9). This solution is called an isotonic
solution. This strength of salt solution is often called normal saline
and is used when replacing an animal�s body fluids or when cells like red blood
cells have to be suspended in fluid.
Diagram 3.9 - Red cells placed in an isotonic solution
Remember - osmosis is a special kind of diffusion. It is the diffusion
of water molecules across a semi-permeable membrane. It is a completely passive
process and requires no energy.
Sometimes it is difficult to remember which way the water molecules move.
Although it is not strictly true in a biological sense, many students use the
phrase �SALT SUCKS� to help them remember which way water moves across
the membrane when there are two solutions of different salt concentrations on
either side.
As we have seen water moves in and out of the cell by osmosis. All water
movement from the intestine into the blood system and between the blood
capillaries and the fluid around the cells (tissue or extra cellular fluid)
takes place by osmosis. Osmosis is also important in the production of
concentrated urine by the kidney.
c) Active transport
When a substance is transported from a low concentration to a high
concentration i.e. uphill against the concentration gradient, energy has to be
used. This is called active transport.
Active transport is important in maintaining different concentrations of the
ions sodium and potassium on either side of the nerve cell membrane. It is also
important for removing valuable molecules such as glucose, amino acids and
sodium ions from the urine.
d) Phagocytosis
Phagocytosis is sometimes called �cell eating�. It is a process that requires
energy and is used by cells to move solid particles like bacteria across the
plasma membrane. Finger-like projections from the plasma membrane surround the
bacteria and engulf them as shown in diagram 3.10. Once within the cell, enzymes
produced by the lysosomes of the cell (described later) destroy the bacteria.
The destruction of bacteria and other foreign substance by white blood cells
by the process of phagocytosis is a vital part of the defense mechanisms of the
body.
Diagram 3.10 - Phagocytosis
e) Pinocytosis
Pinocytosis or �cell drinking� is a very similar process to phagocytosis but
is used by cells to move fluids across the plasma membrane. Most cells carry out
pinocytosis (note the pinocytotic vesicle in diagram 3.3).
f) Exocytosis
Exocytosis is the process by means of which substances formed in the cell are
moved through the plasma membrane into the fluid outside the cell (or
extra-cellular fluid). It occurs in all cells but is most important in secretory
cells (e.g. cells that produce digestive enzymes) and nerve cells.
